Antioxidant



ANTIOXIDANT Paul M. Downey, Nitro, W, Van assignor to Monsanto ChemicalCompany, St. Louis, Mo, a corporation of Delaware No Drawing.Application June 10, 1%0,

Serial No. 167,493

11 Claims. (Cl. 26tl-315) This invention relates to a method ofpreserving a rubber, especially a rubber in the" form of latex, and tothe products thereof.

Latex compositions and particularly natural latex normally contain ananticoagulant and an antiseptic or g'errnicide. Additionally it is'common practice to add an antioxidant for the rubber. All of theseingredients are loosely referred to as preservatives but the presentinvention is concerned only with the antioxidant type of preservative;In the manufactureof. articles from a latex rubber the compounding andvulcanizing ingredients including the antioxidant must be added to' thematrix before the rubber is coagulated. Many otherwise suitableantioxidants cannot be used in latex because of some adverse effect onthe system such: as discoloring or destabilizing the dispersion. Forexample, certain phenolic sulfides are valuable non-discoloringantioxidants for massed rubber but possess the unfortunate disadvantageof thickening latex and some even bring about immediate coagulation. Thepresent in? vention provides a class of non discoloring phenolic sulfideantioxidants which are eminently suitable for addition to an aqueousdispersion of a rubber.

in accordance with the present invention it hasbeen discovered that thiobis (4,6-dialkyl phenols) containing at least four but less than eightcarbon atoms in each. alkyl group are efiective non-discoloringantioxidants for rubber which do not have any adverse effect on thestability of the latex but on the contrary may be added to the latexwithout thickening or coagulating it. It was found that the size, type,number and position of the alkyl groups all influence the compatibilitywith latex. These products may be prepared by reacting a 4,6-dialkylphenol with a sulfur halide. The sulfur chlorides are convenient to useand are readily available for this purpose. Condensation of twomolecular equivalents of the phenol with one molecular equivalent of.

sulfur dichloride produces a mono sulfide or mono thio bis-phenol.Additional sulfur maybe introduced into' the molecule by increasing theratioof sulfur dichloride or by substituting sulfur mono'c'hloride forsulfur dichloride. While the antioxidant effectiveness is not in creasedby increasing the sulfur content of the molecule,

neither is it noticeably decreased until the sulfur ratiois greater thantwo atoms of sulfur for two dialkyl phenol radicals. Whether or 'not adisulfide linkage forms with higher sulfur content is not definitelyknown but the experimental evidence indicates as more prob-- able thatpolymers are formed having more than twophenol groups linked together bysulfur atoms.v Thus the composite reaction products of either sulfurmonochloride or sulfur dichloride probably containsome polymericconstituents and these reaction products are suitable for incorporatinginto latex without further purification than to remove by-p'roduc'thydrogen chloride.

The sulfur would be expected to attachTo the nucleus at the free orthoposition and additional sulfur nited States Patent O 7 2,726,277Patented .Dec. 6, 1955 tions. The term a thio bis (4,6-dialkyl phenol)isused in a generic sense to include the sulfur halide re actionproducts of 4,6-dialkyl phenols generally.- The ratio of the phenolradical to sulfur may vary and in-'- eludes 2:1, 1:1 and even highersulfur ratios.

The reactions may be carried out in" a solvent unaffected by a sulfurhalide orpreferably in the absence of a solvent by direct reactionbetween the phenol and sulfur" halide. It is unnecessary to employ purefrac tions of. the dialkyl phenols. Small amounts of other isomersproviding they are not present in more than a' few percent have notproved deleterious. Thus it is' probable that small amounts of2,6-dialkyl phenols are present in the commercial grades of 2,4-di'alkylphenols although these commercial fractions have proved to be"satisfactory without further purification. The length' of the alkylchain is a critical factor as compounds with less than four or more thanseven carbon. atoms in the alkyl groups cause rapid coagulation oflatex. Thio bis (4,6-di-secondary amyl phenol) is about optimum and ispreferred. So faras is'known this product has never before beendescribed. I

Without limiting the invention the following are recited to illustratethe preparation of the new anti oxidants.

EXAMPLE l 234 grams (1.0 molecular proportion) of diamyl phenolconsisting essentially of 2,4-diamyl. phenolinwhich. the alkyl groupswere predominantly tertiary, were charged into a suitable containerprovided with a stirrer and condenser. 58.8 grams (0.57 molecularpropor-" tion) of sulfur dichloride were added slowIy,v keeping. thetemperature below about 50 C. Stirring was continued for a short timeafter the addition of the sulfur dichloride and the reaction mass thenheatedto 100 C. under partial vacuum. The pressure in the reactor wasgradually reduced to about mm. inorder to distill ofi? any volatileconstituents. In this manner there was produced a dark brown extremelyviscous liquid consisting essentially of thio bis- (4,-6-di-tertiaryamyl phenol).

EXAMPLE 2" 234 grams (1*.0 molecular proportion) of 2,4-di-sefcondaryamyl phenol were charged'into a suitable reactor equippedwith a' stirrerand condenser. 5818' grams (057' molecular proportion) of sulfurdichloride were grad- EXAMPLE 3 The reaction product of sulfurmonochloride was prepared by substituting 77.0 grams of sulfurmonochloride for the 58.8 grams of sulfur dichloride in the" foregoingexample; After removal of the volatile constituents as described, therewas obtained a resinous product con=- taining substantially one atom ofsulfur for each- 2 ,4' di secondary amyl' phenol radical. V

The critical differences brought about by small changes" in chain lengthmay be readily demonstrated by adding the antioxidant'to the latexandfollowingthe change in viscosity with time. For example a typicaldipped goods base stock was compounded comprising:

The stocks were compounded in the manner common to the art by addingzinc oxide, sulfur and accelerator in the form of dispersions to theconcentrated latex. The antioxidants were also added as dispersions, thefollowing procedure proving satisfactory for the preparation of such anantioxidant dispersion: 100 parts of the antioxidant were mixed with 10parts of a 10% solution of ammonium caseinate with moderate stirring and4 parts of oleic acid were then introduced and after mixing about 5minutes, 4 parts of aqueous NaOH was added and finally 82 parts of wateradded very slowly.

' The viscosities of the latex were determined after compounding and atintervals thereafter. For this purpose a Brookfield viscometer was used.The data set forth in the table below are relative figures reported aspercentages of the control stock which contained no antioxidant. Inother words the control stock was arbitrarily assigned a value of 100.

Table II l l i i i of 'I ll Ult ast c ty ens e Stock ig fm in mum. atBreak Elong., at 500% in1bs./in. Percent;

Elongation It will be noted that the antioxidants have no adverse effecton the cure and are eflicient antioxidants.

In addition, samples of the cured stocks were exposed to the rays of an8-1 suniamp. After ten days exposure there was no noticeable diflerencebetween the color of exposed stocks A and C. Stock B was only slightlydarker. In order to obtain a more precise measure of the colorvariation, the light reflected from the surface of the stocks wasmeasured by means of a Photovolt reflectance meter calibrated againstreflectance of standard MgO as 100%. The values are reported as Table IRelative viscosity aiter Antioxidant hrs. hrs. hrs. hrs. hrs. hrs. hrs.hrs. hrs.

None 100 100 100 100 100 100 100 100 100 Thio bis (4,6dl-isopropyiphenol 80 500 1,270 coag. Thio bis (4.6-di-tertiary buty] phenol) 79 51vise. coag. Thio his (4.6-di-tertiary amyl phenol) 72 65 78 81 88 134Thio bis (4,6-di-secondary amyl phenol) 67 55 69 73 77 106 Thto bis(4.6-di-tertiary octylphenoil 79 coag.

Stock (Parts by Weight-Dry Basis) A B C Rubber as 60% latex 100 100 100Zinc oxide 1 1 1 Sulfur" 1.5 1.5 Zinc diethyl dithiocarbamate 0. 5 0. 5Titanium dioxide 20 20 Thin bis (4,6-di-tertiary nmyl phenol) 1 Thio bis(4,6-dl-secondary amyl phenol) 1 Films were cast on glass from the latexcompounds so prepared and were stripped after about 12 hours and driedat room temperature in circulating air. After about 18 hours drying thefilms changed from opaque to translucent indicating dryness and werethen cured by heating for 60 minutes at 100 C. in circulating air.Before testing, the dry films Whether cured or uncured, were conditionedby holding at constant temperature in a desiccator over sulfuric acidfor at least 24 hours. The tensile properties were then measured in theusual manner.

Aging tests were carried out by heating samples of the compounds 9 hoursin a bomb at 121 C. under 80 pounds air pressure per square inch.

per cent of reflectance and are a measure of the whiteness or in otherwords absence of discoloration of the stocks, higher values meaning awhiter stock.

Table III Percent Light Reflectance After 10 days Exposure ToUltraviolet Light Stock A B C Similar tests were carried out in latexsponge stocks comprising:

These ingredients except the zinc oxide and antioxidant were compoundedin known manner and 300 gram portions of the stock frothed using 10grams of 10% ammonium oleate as a dispersing aid. By means of a highspeed mixer they were frothed to six times their original volume and thespeed of the whip reduced to permit the froth to smooth out beforeadding the anti- Table IV Percent Light Reflectance after Exposure to8-1 Lamp (days) Antioxidant None Thio bis (4,6-di-tert. amyl phenol)Thio bis (4,6-d1-sec. amyl phenol) Similar desirable properties areexhibited in synthetic rubber compositions, as for example GR-S latextype III. In this case it is desirable to use a saturated fatty acidsoap in formulating the dispersion of the antioxidant which is added tothe latex so that the composition will be readily compatible. The latexcontains a soap dispersing agent and use of dispersants incompatibletherewith must of course be avoided. A satisfactory formulation is asfollows:

Thio bis (4,6-di-sec. amyl phenol) 100.0 10% ammonium caseinate 20.0 10%NaOH 5.0 10% Ivory soap 20.0 Oleic acid 5.0 Water 140.0 Ethylene glycol10.0

The ethylene glycol is added to the antioxidant at 50-60? and themixture added with rapid stirring to a mixture of the ammoniumcaseinate, oleic acid and Ivory soap.

At this point 50 parts of warm (75-80 C.) water are introduced slowly,then 5 parts of NaOH and finally the remaining water. The composition isthen added to the latex in such amount as to provide the desiredconcentration of antioxidant. The latex containing 1.0% of theantioxidant on the total GR-S solids remained stable for long periodsand after coagulation in the usual manner the elastomer was found to bemore resistant to oxidation than the untreated control. Additionallythere was no discoloration or staining of the elastomer.

In general amounts within the range of 0.5% to 3.0% based on the totalelastomer solids are preferred. While 1.0% is the amount usuallyemployed even better protection may be achieved by increasing theconcentration to 2.0%. The effect of increasing quantities graduallydiminishes. The antioxidant properties are of course similar regardlessof how the antioxidants are incorporated into the rubber or elastomer.Thus the method of treating the rubber may vary. The antioxidant may beadded to the rubber by milling or similar process or applied to thesurface of a mass of crude or vulcanized rubber as well as added to thelatex before coagulation.

What is claimed is: I

1. The method of preserving a rubber which comprises incorporating intothe latex a small amount of a thio bis (4,6-dialltyl phenol) in whicheach alkyl group contains more than three but less than eight carbonatoms containing not more than one sulfur atom per each 4,6- dialkylphenol radical,

.2. The method of preserving a rubber which comprises incorporating intohevea rubber latex a small amount of a thio bis (4,6-dialkyl phenol) inwhich each alkyl group contains more than three carbon atoms but lessthan six containing not more than one sulfur atom per each 4,6- dialkylphenol radical.

3. The method of preserving a rubber which comprises incorporating intothe latex a small amount of a thio bis (4,6-di-sec. amyl phenol)containing not more than one sulfur atom per each 4,6-di-sec. amylphenol radical.

4. The method of preserving a rubber which comprises incorporating intohevea rubber latex 0.5 to 3.0% by weight based on the total elastomersolids of thio bis (4,6-di-sec. amyl phenol).

5. A latex rubber composition having incorporated therein a small amountof a thio bis (4,6-dialkyl phenol) in which each alkyl group containsmore than three but less than eight carbon atoms containing not morethan one sulfur atom per each 4,6-dialkyl phenol radical.

6. A hevea latex rubber composition having incorporated therein a smallamount of a thio bis (4,6-dialkyl phenol) in which each alkyl groupcontains more than three carbon atoms but less than six containing notmore than one sulfur atom per each 4,6-dialkyl phenol radical.

7. A latex rubber composition having incorporated therein a small amountof a thio bis (4,6-di-sec. amyl phenol) containing not more than onesulfur atom per each 4,6-di-sec. amyl phenol radical.

8. A hevea latex rubber composition having incorporated therein 0.5% to3.0% by weight on the total elastomer solids of thio bis (4,6-di-sec.amyl phenol).

9. A hevea latex rubber composition having incorporated therein 0.5 to3.0% by weight on the total elastomer solids of thio bis (4,6-di-tert.butyl phenol).

10. A hevea latex rubber composition having incorporated therein 0.5% to3.0% by weight on the total elastomer solids of thio bis (4,6-di-tert.amyl phenol).

11. An hevea latex rubber composition having incorporated therein asmall amount of thio bis (4,6- diarnyl phenol).

References Cited in the file of this patent UNITED STATES PATENTS1,849,489 Howland Mar. 15, 1932 2,175,082 Hagen Oct. 3, 1939 2,230,542Meinert et al. Feb. 4, 1941 2,310,449 Lightbown et al. Feb. 9, 19432,342,099 Ashley Feb. 22, 1944 2,362,291 Winning Nov. 7, 1944 2,364,338Beaver Dec. 5, 1944 2,370,756 Sibley Mar. 6, 1945 2,380,299 Evans et al.July 10, 1945 2,409,687 Rogers et al. Oct. 22, 1946 2,459,754 AlbertJan. -18, 1949 2,581,919 Albert Jan. 8, 1952 FOREIGN PATENTS 359,045Great Britain Oct. 19, 1931

5. A LATEX RUBBER COMPOSITION HAVING INCORPORATED THEREIN A SMALL AMOUNTOF A THIO BIS (4,6-DIALKYL PHENOL) IN WHICH EACH ALKYL GROUP CONTAINSMORE THAN THREE BUT LESS THAN EIGHT CARBON ATOMS CONTAINING NOT MORETHAN ONE SULFUR ATOM PER EACH 4,6-DIALKYL PHENOL RADICAL.